This invention relates to a mixer, and especially to a mixer comprising micro-electromechanical systems (MEMS) components.
Some radio frequency (RF) devices require frequency references. At present such frequency references are usually made from quartz, which can readily be driven to oscillate in a frequency range of 1-50 MHz. Radio signals, however, may oscillate at a different frequency, such as in the region of 1 to 3 GHz. Hence to provide a radio frequency reference signal from a quartz reference device one has to derive a radio frequency signal from the oscillation of the quartz device. This is usually done by an electronic oscillator and phase-locked loop, providing a “local oscillator”. Conventionally this operates at or near to the RF carrier to be used by the radio frequency device. Such local oscillator systems can be power-hungry and complex. Furthermore, since the typical resonators used in portable wireless communication transceivers utilise off-chip resonator technologies, such as the quartz resonator, these off-chip resonator components contribute to a substantial percentage of portable transceiver area taken up on the circuit board.
It is possible to fabricate MEMS resonators which have radio frequency resonance. MEMS devices involve the integration of mechanical elements and electronics. A MEMS device is normally made on a single silicon substrate. It can be made through microfabrication technology. While the electronics are typically fabricated using integrated circuit process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are typically fabricated using compatible “micromachining” processes, for example, that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
By fabricating MEMS oscillators on a die that comprises the electrically active circuits (such as CMOS amplifiers, analogue to digital converters and logic), the need for off-chip resonator components would be reduced, thus saving power and reducing circuit size.
There is therefore a need to effectively and efficiently implement MEMS oscillators in radio receivers and transmitters.